The present invention relates to a fuel intake system for a supercharged internal combustion engine.
The Japanese Laid-open patent publication No. 55-137314, laid open to public inspection on Oct. 27, 1980, discloses a fuel intake system for a supercharged engine which comprises a primary intake system and an auxiliary intake system including a supercharger driven by a drive unit, for example, the engine. This prior art fuel intake system is so designed that, when and so long as the load on the engine is smaller than a predetermined value, only the primary intake system is brought into operation to supply a combustible air-fuel mixture into the engine through an associated primary intake port, but when and so long as the load on the engine is larger than the predetermined value and at least during the period in which the engine undergoes the compression stroke, the auxiliary intake system is brought into operation together with the primary intake system to allow a supercharged air from the supercharger to be supplied into the engine through an associated auxiliary intake port.
This prior art system appears advantageous in that, since the supercharger is driven by the engine in contrast to a turbocharger driven by exhaust gases emitted from the engine, the supply of the supercharged air into the engine would not be short of the required amount even at a low speed engine operating condition during which the flow of the exhaust gases through the exhaust manifold tends to be retarded.
This publication also discloses the use of an auxiliary throttle valve, situated in the auxiliary intake passage downstream of the supercharger with respect to the direction of flow of the supercharged air towards the engine, and also a relief passage having a relief valve and bypassing the supercharger. According to this prior art system, the supercharger is continuously driven by the drive unit, for example, the engine, irrespective of the engine operating condition, and therefore, the relief passage is brought into operation during the period in which no supply of the supercharged air into the engine is required. Specifically, during that period, the supercharged air emerging from the supercharger is relieved back to the supercharger through the relief valve without flowing past the auxiliary throttle valve.
The fact that the supercharger is continuously driven by the engine even during the period in which no supply of the supercharged air is required is disadvantageous in that the engine is unnecessarily loaded and, accordingly, the engine tends to consume the relatively increased amount of fuel.
The prior art system now under discussion has an additional disadvantage. That is, if the timing at which the auxiliary throttle valve, which is seemingly effective to avoid any possible back-flow of the air-fuel mixture from the engine into the auxiliary intake passage, is to be controlled deviates from its timed relationship to the start of the supercharger, i.e., the change in pressure of the supercharged air in the auxiliary intake passage downstream of the supercharger, not only may an abrupt change in engine power output occur, but the servicing life of the supercharger may also be adversely affected because it is drivingly coupled to the engine.